Magnetic field sensitivity of PMT

Hi Joerg, I know only a little. But I think the mu-metal shields on PMT's are there so that if you change the orientation of the pmt in the earth's field then the gain won't change. (I've never tried to measure the effect, but I've used PMT's with mu-metal shields.) So at a guess if you keep the B-field well below that of the Earth's maybe you'll be fine. Toroidal inductors?

I think you give commercial socket makers too much credit. We have some hamamatsu sockets that put varying amounts of (I think) 40 kHz crud from the socket C-W multiplier onto the anode. We use it in photon counting mode and the crud is just an annoyance, not a real problem. What mode will you be using?

George H.

On a sunny day (Mon, 21 Dec 2015 09:23:46 -0800) it happened Joerg wrote in :

Depends on the PMT, how much gain, but I think an inch is very little. Depends on the transformer too. My setup:

PMT socket on the right Can do up to 2 kV, but now runs a simple Russian FEU35 at much lower voltage, say 500 to 600 V.

At that point I have no problems.. But I have a strong signal from a scintillator crystal.

AND it is not a switchmode, but a SINE wave generator. smft

Probably even less because an AC field creates a signal in the tube.

Unfortunately not in the cards, this is all on a very small budget. I have to live with what's available and right now only CCFL transformers are left. For a while. Soon they'll also go lalaland because monitors are migrating to LED backlighting.

That's what I also assumed, I always wondered how that could ever work. Well, in this case it didn't which is why they want a new kind of circuit that performs better.

I don't have much information about the use, just that it all has to be very stable, clean and low noise. Unfortunately that also means large.

--
Regards, Joerg 

http://www.analogconsultants.com/

A transformer fastened with rope, cool. That's a new one to me. I've used lacing cord but not packaging cord (yet).

The problem in my case is that I don't have those 11 centimeters.

I hope Vladimir's guys don't know about this ...

Mine will be very close to a sine wave so no worries from harmonics.

--
Regards, Joerg 

http://www.analogconsultants.com/

So are you using HV and a resistor chain or a cockroft-walton type thing? Are you sure it's the B-field in the pmt that is causing the problems? Phil H. had a thread a week or two ago about PMT drives. There they were running the pmt at fairly high currents, and I think he mostly made the voltages sources driving the last few dynodes stiffer.

I think he had a link to a good pmt paper.

George H.

On a sunny day (Mon, 21 Dec 2015 11:11:14 -0800) it happened Joerg wrote in :

That is actually a rubber band, and it is just fitting component placing. IIRC it is now fixed with a tie-ribbon.

The problem is that you always have a problem, why not sell it ;-)

I am more worried about demonrats.

You may like this movie:

That's all stable and very stiff. Phil is the guru on this stuff.

I must have missed the whole thread, was busy with an x-ray design.

Right now I am only concerned about how much of the remaining magnetic field could cause trouble. Since I don't have a CCFL transformer here I can't really measure it. So I just wanted to know whether it's serious or way down below the noise.

One issue with split boards is that if you have the HV gen on a separate board, some measurement hooks on the tube socket and then for some reason the ground return becomes loose ... bzzzt.

[...]

--
Regards, Joerg 

http://www.analogconsultants.com/

You can't sell a problem :-)

Anyhow, there is always a solution, I am just looking for the simplest one.

Me, too.

It is blocked on my PC, can't see it. Do you have it on Youtube also?

--
Regards, Joerg 

http://www.analogconsultants.com/

On a sunny day (Mon, 21 Dec 2015 12:33:07 -0800) it happened Joerg wrote in :

I knew :-)

(google groups sorry.) Third order IMD in photomultipliers. He's got a link to a pmt book on his web site in a post on November 23rd.

(Or just drop him an email and I'm sure he'll send you the link)

I would guess small, but there's a 50% chance I'm wrong :^)

George H.

Interesting. I didn't know PMT's were used in astrophysics.

Quote page 30 "... and indeed are adversely affected by magnetic fields of even a few Gauss". I think the earth magnetic field is half a Gauss. So it's really in the same ballpark.

We are in good contact about this :-)

Like the weather forecast on TV :-)

Friday they announce one humdinger of a rain storm for Sunday and then I and a buddy enjoyed a very nice and completely dry bike ride.

[...]

--
Regards, Joerg 

http://www.analogconsultants.com/

No.

A lot depends on the dynode structure.

Classically there are three.

Fast focussed, which you mostly find in fast tubes, though the cheap and ub iquitous side-view tubes also seem to use it.

Venetian blind, and box and grid.

In the fast focussed tubes, the electron trajectories are supposed to be se t up by the electric fields between the dynodes, and a significant magnetic field can screw them up.

In Venetian blind tubes, the electron trajectories are short, and non-criti cal, so magnetic fields mostly don't matter - though the space between the photocathode an the first dynode may be an exception, if the magnetic field is big enough to force most of the photoelectrons away fro the first dynod e, as was exploited by a scheme that put a fairly potent ring magnet in fro nt of the photocathode, which generated a field that meant that only the ph oto-electrons from the centre of the photocathode (the only area illuminate d) made it to the first dynode, killing the dark current from the rest of t he photocathode.

Box and grid multiplier structures are supposed to be fairly insensitive to magnetic fields, but since I never used a box-and-grid tube, I never got t hat interested.

--
Bill Sloman, Sydney

Instead of mu-metal, you can probably just use aluminum. The relatively high frequency of the switchmode devices around today means that a little conductive shielding will block the AC magnetic field well.

At 50 kHz, skin depth in aluminum is 0.36 mm.

Your CCFL transformer is E-E core, I suspect, and will self-shield fairly well if you don't get it close to saturation.

After what George wrote about sub-Gauss fields already being troublesome it sounds like it could be like roaring through a Mozart concerto on a Harley.

It is a side window tube. However, I've resorted to splitting it up so the noisy stuff is far away. I added a sense circuit into one of the ground wires so it all shuts down when it comes off for some reason.

--
Regards, Joerg 

http://www.analogconsultants.com/

True, but normally 10mil foil yields only around 25db reduction for the magnetic field at 50kHz. Not much. That's about 1/4mm. A more sturdy can should do better but considering that things have to be well below a Gauss I decided against placing the transformer up there. It'll be more work but less chance of egg in the face.

--
Regards, Joerg 

http://www.analogconsultants.com/

Oh, I was thinking more of 3mm wall aluminum tube around the photomultiplier; with a slit, of course. There is usually a light-blocking shroud anyhow.

Weather forecasts are generally better on the East Coast, no doubt because there's a lot more data available.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net

Hmm I can't get Phil's link to work. But yeah something like 0.5 Gauss. The guy I knew who was using the mu-metal shield wanted to get some sort of absolute calibration for the pmt... like the 1 or 2% level. (That turns out to be a PITA.)

OK good, then everything should be well in hand.

We had "thunder snow" here on Friday. About 6" of lake effect, with these diffuse white flashes lighting up the sky as I drove home. All rain now and into the 60's later in the week.

George H.

The R10699 datasheet doesn't say anything about magnetic field sensitivity. I already returned the one the client sent for testing, or I'd measure it.

My guess would be that it would cause some amount of phase modulation due to transit time effects as much as gain changes.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net